The present invention relates generally to the field of manufacture of optical data storage disks, and in particular, to a method and assembly for reducing or eliminating an increased thickness that occurs at the outer edge of an optical disk substrate as a result of the substrate molding process (otherwise known as the xe2x80x9cedge wedgexe2x80x9d effect).
Data storage disks are produced using a disk replication process. A master disk is made having a desired surface relief pattern formed therein. The surface relief pattern is typically created using an exposure step (e.g., by laser recording) and a subsequent development step. The master is used to make a stamper, which in turn is used to stamp out replicas in the form of replica disk substrates as part of a disk molding process. As such, the surface relief pattern, information and precision of a single master can be transferred into many inexpensive replica disk substrates.
Conventional mold assemblies typically include a fixed side and a moving side. The stamper is typically attached to either or both sides of the mold assembly for replicating a desired surface relief pattern (i.e., lands, grooves and/or pits) into the replica disk substrate. A movable gate cut may be provided for cutting a central opening in the replica disk substrates. The stamper may be secured to the moving side using an inner holder and outer holder, wherein the inner holder and outer holder fit over the stamper. Several more tooling parts may be located at the center of the mold assembly to assist in ejection of the component.
During the disk molding process, a resin, typically optical grade polycarbonate, is forced in through a sprue channel into a substrate cavity within the mold assembly to form the replica disk substrate. The surface relief pattern or formatted surface is replicated in the replica disk substrate by the stamper as the cavity is filled. After filling, the gate cut is brought forward to cut a center hole in the replica disk substrate. After the replica disk has sufficiently cooled, the mold assembly is opened and the gate cut and a product eject may be brought forward for ejecting the formatted replica disk substrate off of the stamper. The inner holder and outer holder may be removable to allow changeout of the stamper.
In injection-compression molding, while the resin is forced into the substrate cavity of the mold assembly by the molding press, injection pressure overcomes clamp force causing mold to open a small amount (commonly termed xe2x80x9cmold blowxe2x80x9d). Pressure is then increased to the mold assembly to clamp the mold shut, forcing the resin into the microscopic surface relief pattern of the stamper (which contains the reverse image of the desired replica disk surface relief pattern). Thus, the above process is commonly termed xe2x80x9cinjection compressionxe2x80x9d or xe2x80x9cmicro-coiningxe2x80x9d.
For disk formats utilized in flying head applications, as disk capacity increases the design tolerances for the desired surface relief pattern become more critical. For high capacity disks the flying heads may be required to pass closer to the disk substrate, requiring tighter disk specifications, including a reduction or elimination of disk surface geometry imperfections.
One such disk surface geometry imperfection is the thickness increase that has been consistently seen at the outer edge of a typical polymeric optical disk substrate. This phenomenon has been given the name xe2x80x9cedge wedgexe2x80x9d or xe2x80x9cski jumpxe2x80x9d effect. This xe2x80x9cedge wedgexe2x80x9d is shown schematically in prior art FIG. 1 and FIG. 2. xe2x80x9cEdge wedgexe2x80x9d causes problems in a hard disk-type system where a read/write head is designed to fly as close as possible (i.e., on the order of 1-5 micro-inches) to the surface of the media substrate. For example, one typical polycarbonate disk substrate has an average thickness of about 2 mm (as shown at T1), and a radius of 65 mm. The xe2x80x9cedge wedgexe2x80x9d effect is primarily seen at the outer radius region of the polycarbonate disk between 62 mm and 65 mm, where the maximum substrate thickness (i.e., bump height) T2 at radius 65 mm is approximately 10-20 microns thicker than the substrate thickness at radius 63 mm. When the bump height differential (T2xe2x88x92T1) divided by the average thickness (T1) exceeds 0.01 (1 percent), read/write flyability problems are often encountered.
The xe2x80x9cedge wedgexe2x80x9d phenomenon can be attributed to many factors. During cooling of the disk substrate in the mold, the plastic xe2x80x9cfreezesxe2x80x9d at different rates in different radii of the part. The outer edge of the disk substrate freezes through the thickness faster due to its contact with the cold outer holder. Other factors include the tendency of the disk substrate material molecules to be in substantial radial alignment near the center of the disk substrate, and relatively misaligned near the outer edge due to the mold filling process. All of these factors result in the outer edge of the disk substrate exhibiting a greater thickness than the remainder of the disk substrate.
The xe2x80x9cedge wedgexe2x80x9d phenomenon can be further described as follows. When the optical disk substrate is molded in the micro-coining process described earlier, the densification that is associated with the cooling plastic is accommodated through a corresponding reduction in the mold cavity size (as opposed to reduction in mold cavity pressure as in conventional injection molding). During the filling phase, the mold halves are forced slightly apart by the fluid pressure applied from the injection unit. As the plastic in the mold cools, it shrinks, and the mold halves translate into closer proximity as the press maintains a constant clamp force or pressure on the solidifying melt. The part will freeze through the thickness at slightly different rates at different radii. Regions that are frozen fully through early will do so while the mold is blown to a greater extent or whilst the cavity z-dimension is larger in the earlier phases of the micro-coining molding process. These fully frozen regions will then strain due to clamp force in an elastic fashion (meaning that the solid material will spring back upon release of applied force). Regions that remain liquid at the center will strain in a viscous fashion (this is non-recoverable strain) and will continue to shrink in size or density as they more slowly solidify and eventually take on a thickness of a smaller cavity dimension from later in the coining/cooling process. Therefore, after the clamping force is removed, the early-freezing regions (outer circumference areas) will spring back to a larger thickness than those areas that froze completely through later in the process (the inner disk area).
For a traditional optical disk, where information is stored xe2x80x9csubstrate incidentxe2x80x9d the xe2x80x9cedge wedgexe2x80x9d effect does not present a major problem. In substrate incident applications, a transparent protective layer covers the information layer of the disk. An optical disk player including a laser light source positioned away from the disk surface, focuses a laser beam through the protective layer at the information layer to access (i.e., read) the data stored on the disk. However, for xe2x80x9cflying headxe2x80x9d applications where information is stored on a disk surface (i.e., where information is stored xe2x80x9cair incidentxe2x80x9d), a read/write head is flying 1-2 micro-inches above the substrate surface. The xe2x80x9cedge wedgexe2x80x9d phenomena is associated with a loss of flyability of the read/write head where the outer edge of the head comes into contact with the rising surface of the media substrate, resulting in a xe2x80x9chead crashxe2x80x9d if the head were allowed to fly over the outer portion of the disk. The outer edge of the disk is unusable for data storage, since the curvature of the surface becomes too great to provide a functional air bearing between the head and the surface of the disk. This limits the capacity, functionality and robustness of the disk data storage system.
Unfortunately, the outer circumference of the disk substrate where the xe2x80x9cedge wedgexe2x80x9d effect occurs is also the most desirable area for data storage. This outer circumference provides a large area for data storage since the data tracks are larger. Therefore, the need exists to eliminate the xe2x80x9cedge wedgexe2x80x9d to prevent disk crashes and to increase the useable area of the disk.
The present invention discloses an optical disk exhibiting no detrimental thickness increase (edge wedge effect or curvature) that arises at the outer diameter of an optical disk substrate during a typical injection molding manufacturing process, and an apparatus and method for making such a disk.
The present invention provides an optical disk for use with an optical disk player, where the data on the optical disk is stored air incident. This optical disk includes a disk substrate made from a molded polymeric material. The disk substrate has a first major surface, a second major surface, and an outer edge. The first major surface of the optical disk includes a data region having an intermediate portion and an outer portion. The outer portion extends close to the outer edge of the optical disk. The data region is defined by a plurality of lands and grooves, where the disk substrate has a thickness defined by the distance between the lands and the second major surface. The optical disk also includes an information layer covering the data region. In the present invention, the thickness of the intermediate portion of the data region is substantially equal to the thickness of the outer portion of the data region such that the outer portion of the data region is capable of being used by the optical disk player.
In a preferred embodiment of the present invention, the molded polymeric material is a polycarbonate or a polycarbonate blend. In order to prevent a flying read/write head from crashing on the surface of the optical disk, the thickness of the intermediate portion and the thickness of the outer portion of the optical disk varies less that 0.10 micron per millimeter proceeding radially from the center axis of the disk substrate. In one embodiment of the present invention, the outer portion of the optical disk extends radially from approximately 5 millimeters in from the outer edge of the disk substrate to the outer edge of the disk substrate, where the disk substrate has a diameter of between 120 and 130 millimeters.
The present invention also provides a disk molding apparatus for forming an optical disk in a disk molding process, wherein the apparatus reduces the edge wedge effect in the molded optical disk. In a first embodiment, the disk molding apparatus includes a disk substrate cavity for forming a disk substrate. The disk substrate cavity has a first major surface, a second major surface which opposes the first major surface, and an outer edge. The disk molding apparatus also includes a channel mechanism connected with the disk substrate cavity for allowing disk molding material to enter the disk substrate cavity. The disk molding apparatus further includes a stamper located on one side of the disk substrate cavity for forming a formatted surface relief pattern in the disk substrate. Finally, the disk molding apparatus also includes a thermal inhibiting mechanism located around the outer edge of the optical disk. This thermal inhibiting mechanism inhibits heat flow from the disk substrate during the cooling of the disk molding material to form the disk substrate.
The thermal inhibiting mechanism of the first embodiment includes an outer holder, wherein the outer holder removably secures the stamper to the first major surface. In one preferred embodiment, the outer holder is made of low thermoconductivity titanium. In another preferred embodiment, the outer holder has two ring members, wherein a low thermoconductivity ceramic member is retained between the two ring members, and a portion of the ceramic member extends from the two ring members for retaining the stamper against the first major surface.
In one embodiment, the thermal inhibiting mechanism of the present invention is a heating mechanism, where the heating mechanism heats the outer holder during the disk molding process to a temperature sufficient to create a smaller temperature differential between the disk substrate and the outer holder, reducing the heat transfer between the disk substrate and the outer holder. This heating mechanism of the present invention has several embodiments, including: a resistive heater placed within a channel of the outer holder; heated water circulating through the channel of the outer holder; heated oil circulating through the channel of the outer holder; a film resistive heater coupled to an outer surface of the outer holder; and an induction heater positioned external to the outer holder.
In a second embodiment of the present invention, the disk molding apparatus has a disk substrate cavity which includes a defined wedge containment area where the wedge is directed into during the injection molding process. The disk substrate cavity incorporating the wedge containment area has a surface area less than the surface area of a conventional disk substrate cavity without the wedge containment area. By directing the wedge into a confined area, the area of the disk substrate affected by the edge wedge effect is reduced, resulting in a greater usable data storage area within the optical disk.
In a third embodiment of the present invention, the disk molding apparatus includes a disk substrate cavity for forming a disk substrate. The disk substrate cavity includes a first major surface, a second major surface opposite the first major surface, and an outer edge. The disk molding apparatus also includes a channel mechanism in fluid communication with the disk substrate cavity which allows disk molding material to enter the disk substrate cavity. Finally, the disk molding apparatus includes a stamper, having an information surface and a back surface. The information surface of the stamper forms the first major surface of the disk substrate cavity, and produces a formatted surface relief pattern in the disk substrate during the molding process. Also, during the molding process, the stamper forms a shape which counters the molding edge wedge effect.
In order to counter the edge wedge effect during the molding process, the back surface of the stamper is electroplated with a nickel lip around the outside perimeter, such that as pressure is applied to the back surface of the stamper, the stamper flexes in a concave fashion producing a disk substrate cavity thickness which is narrower at the outside perimeter and wider in all other areas, thus creating an anti-wedge region within the disk substrate cavity. In a preferred embodiment, the nickel lip around the outside perimeter of the back side of the stamper is approximately 3 mm wide, and approximately 15 microns thick. The resultant anti-wedge region within the disk substrate cavity is approximately 15 microns narrower than all other areas of the disk substrate cavity.
The present invention also discloses methods for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. The first such method begins by injecting molding material into a disk substrate cavity via a channel mechanism. The disk substrate cavity includes a first major surface, a second major surface opposite the first major surface, and an outer edge. Next, a thermal inhibiting mechanism located about the outer edge of the disk substrate cavity inhibits the escape of heat in the radial direction from the disk substrate during the cooling of the disk molding material. The thermal inhibiting mechanism includes a low thermal conductivity outer holder. In one preferred embodiment the outer holder is constructed of titanium. In another preferred embodiment, the outer holder includes a ceramic member which contacts the disk substrate during the molding process. In yet another embodiment, the outer holder includes a heating mechanism, wherein the heating mechanism heats the outer holder during the disk molding process to a temperature sufficient to create a smaller temperature differential between the disk substrate and the outer holder, thus reducing the heat transfer between the disk substrate and the outer holder.
The present invention also discloses a second method for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. In this second method, the disk molding process utilizes a disk substrate cavity having a defined wedge containment area located at the outer perimeter of the disk substrate cavity. Initially, disk molding material is injected into a disk substrate cavity via a channel mechanism. Next, the disk molding material in the disk substrate cavity is compressed such that the disk molding material at the outer perimeter of the disk substrate cavity flows into the defined wedge containment area. The disk molding material is then cooled such that the optical disk is formed within the wedge containment mold. In the resultant disk, the unusable surface area is minimized.
The present invention also discloses a third method for forming an optical disk in a disk molding process which reduces the edge wedge effect in the molded optical disk. This third method utilizes a disk substrate cavity having a first major surface, a second major surface opposite the first major surface and an outer edge. This method also utilizes a stamper having an information surface and a back surface. The information surface of the stamper forms the first major surface of the disk substrate cavity. The back surface of the optical stamper is electroplated with a nickel lip at the outside perimeter.
This third method begins by injecting molten disk molding material into the disk substrate cavity. Next, the disk molding material is compressed in the disk substrate cavity. As pressure is applied to the stamper, a formatted surface relief pattern is formed in the disk substrate from the information surface of the stamper. Also, as pressure is applied to the back surface of the stamper, the stamper flexes to form an anti-wedge region in the disk substrate cavity which counters the molding edge wedge effect during the disk molding process. In a preferred embodiment, the nickel lip around the outside perimeter of the back side of the stamper is approximately 3 mm wide and 15 microns thick. The resultant thickness of anti-wedge region in the cavity is approximately 15 microns less than in all other areas of the cavity.